Method of preventing etch on cast iron in plating baths



Oct. 31, 1967 5501 3,350,287

METHOD OF PREVENTING ETCHON CAST IRON IN PLATING BATHS Filed July 1, 1965 25 24 AUXiLIARY POWER 1 SUPPLY 23 1 f MAIN Is as POWER SUPPLY 1 ii a- 1 N T lol'fl v 2o 2o I24 ,I2 4 z v d g United, States Patent 3,350,287 METHOD OF PREVENTING ETCH ON CAST IRON IN PLATING BATHS Ram Dev Bedi, Oak Park, Mich., assignor to M & T

Chemicals Inc., New York, N.Y., a corporation of Delaware Filed July 1, 1963, Ser. No. 291,986 7 Claims, (Cl. 204-147) This invention relates to plating and more particularly to a technique for preventing etching of cast iron cathodes at cathodic areas of lowcurrent density.

As is well known to those skilled in the art, electroplating of various metals, typically nickel, copper, tin, zinc, or chromium may be effected in baths of varying acidity and composition; many of these baths may contain chloride or fluoride together with oxidizing agents which may be an integral part of the bath or present as an additive. During plating of basis metals in such baths, it is necessary to control current density over the areas to be plated. Because of differences in area or geometry or accessibility, there may be portions of the cathode which have a current density low enough so that plating is not expected to and in fact does not occur. It has been found, particularly when the bath is acid and contains e.g. chloride or fluoride ions together with oxidizing agents, that in these low current density areas, etching is a problem. Specifically, the surface of these areas may be corroded during plating operations sufliciently to alter the dimensions substantially and/ or to spoil the appearance of the finished article. This is particularly true on cast iron if the low current density areas become anodic. The problem of etching maybe present in various baths, typified by chromium plating baths, nickel plating baths, =acid copper baths, acid tin baths, acid zinc baths containing halides or halide-complexes and frequently containing oxidizing agents, etc.; for purposes of convenience, reference will hereinafter be made to chromium plating baths.

As is well known to those skilled in the art, chromium plating for example may be effected by use of a bath containing chromic acid and sulfate optionally together with other compounds which may be employed to effect various desirable results. Typical of these optional additive compounds may be fluorides or fluoride complexes.

Other illustrative chromium plating systems may include soluble catalyst systems containing e.g. chromic acid and sulfate such as sulfuric acid in amount sulficient to give the desired concentration of sulfate ion, or self-regulating baths, typically those containing silicofluorides together with sulfate.

During plating of chromium onto cast iron from baths typified by the foregoing, it is common to operate at a temperature which may vary depending upon the current density, speed of plating, and type of plate desired. Commonly however the temperature of operation may be 34 C. 72 C. and typically 48 C.-63 C. The cathodic current density may preferably be controlled to fall in the range of 8-100, and typically 12-50 a.s.d. (i.e. amperesper square decimeter) on the selected areas of high current density whereon plating may occur. However, because of the irregular shape of many pieces which are to be chromium plated, it is not possible to maintain uniform desired current density over the entire piece. If the current be set to provide a current density as noted, there will be places, typically interior portions, end portions, back portions, or crevices where the current density may -be c0nsiderably. lower, for example 1.6 or less and frequently 0.3-1.0 a.s.d. Such areas may even become anodic under certain conditions.

It has been found that these areas of low current density of the cast iron cathode are etched during plating and especially so when the bath is a chromium plating bath which contains fluoride or silicofluoride ions which may be present in self-regulating high speed baths, soluble catalyst baths, or sparingly soluble catalyst chromium plating systems. In order to to eliminate this etching in chromium plating systems, it has heretofore been common to coat with tapes or waxes those portions of the cathode where low current density is anticipated. The use of such techniques is time consuming and expensive. Tapes or waxes must be carefully placed and removed, since any holes or spaces (including edge areas which may be undermined) in the protective covering will allow severe etching of the so-exposed metal to take place. Such tapes or waxes may be expensive to purchase and apply. Various other attempts to minimize low current density etching of cathodes in the noted plating baths have not been uniformly successful and there is today no economical etch perventive system which is completely satisfactory.

It is an object of this invention to provide a technique for preventing undesirable etching of cast iron in plating operations including e.g. chromium and nickel plating operations. Other objects will be apparent to those skilled in the art on inspection of the following description.

In'accordance with certain of its aspects, the process of this invention for electroplating a plate onto a cast iron cathode having areas of low cathode current density whereon plating does not occur and selected areas of high cathode current density comprises placing an auxiliary electrode adjacent to the cathode areas of lowcathode current density whereon plating does not occur, maintaining said low current density areas of said cathode uniformly cathodic with respect to said auxiliary electrode, and plating the plate onto said selected areas of the surface of said cathode, the said cathode areas of low current density remaining substantially free. of etching during said plating.

Typical of the plate metal with which the process of this invention may find use may be the aforementioned chromium plate including hard, chromium plate. This invention will be especially suitable for use with the noted chromium plating baths, including the, soluble or sparingly soluble catalyst-containing chromium plating baths, since it is with these baths that the problems of etching may be most severe because of the presence of chloride, fluoride, or silico-fluoride ions in the baths.

The cathodes which may be used in the practice of this invention may be those cast iron articles upon which a plate, typically a chromium plate is desired. Cast iron, as is well known, includes iron commonly containing carbon in amount greater than about 1.7 (typically 2%- 6%) and may include added alloying metals. This metal may be characterized by susceptibility to etching at low current density areasduring, e.g., chromium plating particularly when the baths employed are fluoride-containing chromium plating baths, or if the current actually becomes anodic on parts of these areas.

In practice of this invention, the auxiliary electrode may be placed adjacent to the cathode areas of low current density. Such areas, as will be apparent to those skilled in the art, may be e.g. rear or recessed areas, of the cathode or inside areas of the cathode. Preferably the auxiliary electrode may be placed quite close to the low current density cathode areas notedand as far away from the high current density cathode areas as is'conveniently possible. The auxiliary electrode may preferably be shaped to permit positioning close to the noted areas. Preferably it may have the same configuration as .the areas adjacent to which it is to be positioned.

The auxiliary electrode may preferably beone which-is inert to the electrolyte solution. It may preferably be lead, lead alloys including lead-antimony, lead-tin, etc.

Preferably the auxiliary. electrode may be. maintained anodic to the lower current density areas of the cathode so that the current may fiow to the low current density areas of the plating cathode from the auxiliary electrode thus making the low current density areas uniformly cathodic, even though not at a high enough current density to cause plating. This may be effected by placing the auxiliary electrode in an auxiliary circuit which includes an auxiliary power supply which is electrically connected to the cathode through a DC. ammeter which may measure the direction and flow of current in the auxiliary circuit. The auxiliary power supply may be adjusted or set so that the flow of current in the chromium plating solution may be from the auxiliary electrode to the low current density area of the cathode. This insures that the low current density areas may be protected from etching by being made cathodic at controlled current density, typically 0.1-9 a.s.d., preferably 46 a.s.d. The auxiliary power will preferably be characterized by high internal impedance.

During plating in practice of this invention, the plating current density on the cathode may vary, in case of chromium plating on cast iron, from about 9.5 a.s.d. to 60 a.s.d.

In an illustrative embodiment of the process of this invention, as shown in the attached drawing, a plating solution 10 may be maintained in vessel 11 which contains a cylindrical cast iron cathode piece generally designated 12 to be electroplated on the outside surface 13. Plating anodes 14 and 15, also suspended in solution 10 by standard means similar to that supporting cathode 12, may be in a plating circuit which includes conductors 16, joined to anodes 14 and 15, main power supply 17, and conductor 18 joined to cathode 12. There may also be suspended in the solution 10 the auxiliary electrode generally designated 19 which preferably in this embodiment may be concentric to the low current density areas 20 of cathode 12. The auxiliary electrode 19 may be in an auxiliary circuit which may include conductor 23, ammeter 24, voltmeter 31, auxiliary power supply 25, and conductor 26; the latter may be connected as shown to conductor 18.

In this illustrative example, a piece of cast iron pipe 19 cm. long, cm. inside diameter, and 6 cm. outside diameter was chromium plated in a bath containing 225 g./l. chromic acid, 2.2 g./l. silicofluoride ion (added as K SiF and 1.08 g./l. sulfate ion (added as strontium sulfate). During the period of 120 minutes, the cast iron cathode 12 received a chromium deposit of 125 microns when plated at a current density of 50 a.s.d.

The polarizing current density, generated by the auxiliary power supply, at the low current density areas 20 of the cathode piece 12 was 8.5 a.s.d., which is generally insufficient to deposit chromium on cast iron.

The inside surface 20 of the cathode was found to be completely free of etching and had substantially nonexistent etch band markings.

In a comparable (standard) chromium plating operation, without the auxiliary electrode 19, the inside surface 20 of the cathode was found to be badly etched and had substantial etch bands at each end.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modification and variations as come within the scope of the appended claims.

I claim:

1. The process for electroplating a plate onto a cast iron cathode having areas of low cathode current density whereon plating does not occur and selected areas of high cathode current density which comprises placing an auxiliary electrode adjacent to the cathode areas of low cathode cur-rent density whereon plating does not occur, electrically connecting said auxiliary electrode to said cathode, maintaining said low current density areas of said cathode uniformly cathodic with respect to said auxiliary electrode, and plating the plate onto said selected areas of the surface of said cathode, the said cathode areas of low current density remaining substantially free of etching during said plating.

2. The process for electroplating a plate onto a cast iron cathode having areas of low cathode current density whereon plating does not occur and selected areas of high cathode current density as claimed in claim 1 wherein the auxiliary electrode is lead.

3. The process for electroplating a plate onto a cast iron cathode having areas of low cathode current density whereon plating does not occur and selected areas of high cathode current density as claimed in claim 1 wherein the cathodic current density applied from an auxiliary power supply onto the low current density areas of the cathode is 0.1 a.s.d. to 9 a.s.d.

4. The process for electroplating a plate onto a cast iron cathode having areas of low cathode current density whereon plating does not occur and selected areas of high cathode current density as claimed in claim 1 wherein the cathodic current density applied from an auxiliary power supply onto the low current density areas of the cathode is 4 a.s.d. to 6 a.s.d.

5. The process for electroplating a chromium plate onto a cast iron cathode having areas of low cathode current density whereon chromium plating does not occur and selected areas of high cathode current density which comprises placing an auxiliary electrode adjacent to the cathode areas of low cathode current density whereon chromium plating does not occur, electrically connecting said auxiliary electrode to said cathode, maintaining said low current density areas of said cathode uniformly cathodic with respect to said auxiliary electrode, and plating the chromium plate onto said selected areas of the surface of said cathode, the said cathode areas of low current density remaining substantially free of etching during said chromium plating.

'6. The process for electroplating a chromium plate onto a cast iron cathode having areas of low cathode current density whereon chromium plating does not occur and selected areas of high cathode current density which comprises placing an auxiliary lead electrode adjacent to the cathode areas of low cathode current density whereon chromium plating does not occur, electrically connecting said auxiliary electrode through an auxiliary power supply to said cathode, maintaining said low current density areas of said cathode uniformly cathodic with respect to said auxiliary electrode, and plating the chromium plate onto said selected areas of the surface of said cathode, the said cathode areas of low current density remaining substantially free of etching during said chromium plating.

7. The process for electroplating a plate onto a cast iron cathode having areas of low cathode current density whereon plating does not occur and selected areas of high cathode current density as claimed in claim 6 wherein said electroplating is chromium plating from a bath containing chromic acid, sulfate ion, and fluoride ion.

References Cited UNITED STATES PATENTS 485,343 11/1892 Fletcher.

499,726 6/ 1893 Fletcher 204231 1,055,327 3/1913 Hering 204196 1,465,034 11/ 1921 Antisell 204231 2,508,523 5/1950 Krebs 204--l96 FOREIGN PATENTS 924,180 2/1955 Germany. 1,149,893 1/1958 France.

OTHER REFERENCES Hull: Metal Finishing, September 1944, page 545.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

T. T-UNG, Assistant Examiner. 

1. THE PROCES FOR ELECTROPLATING A PLATE ONTO A CAST IRON CATHODE HAVING AREAS OF LOW CATHODE CURRENT DENSITY WHEREON PLATING DOES NOT OCCUR AND SELECTED AREAS OF HIGH CATHODE CURRENT DENSITY WHICH COMPRISES PLACING AN AUXILIARY ELECTRODE ADJACENT TO THE CATHODE AREAS OF LOW CATHODE CURRENT DENSITY WHEREON PLATING DOES NOT OCCUR, ELECTRICALLY CONNECTING SAID AUXILIARY ELECTRODE TO SAID CATHODE, MAINTAINING SAID LOW CURRENT DENSITY AREAS OF SAID CATHODE UNIFORMLY CATHODIC WITH RESPECT TO SAID AUXILIARY ELECTRODE, AND PLATINGD THE PLATE ONTO SAID SELECTED AREAS OF THE SURFACE OF SAID CATHODE, THE SAID CATHODE AREAS OF LOW CURRENT DENSITY REMAINING SUBSTANTIALLY FREE OF ETCHING DURING SAID PLATING. 